A GM must be a good talent evaluator. It takes no such talent to notice that Cousins is a star. What takes talent is to pluck an unnoticed young player from anonymity and make him bloom. Olshey tried to do this with Ed Davis, Aminu, Turner, etc., but they aren't good enough. The guys who are good enough (Lillard and McCollum) were conventional wisdom for where they were in the draft, not genius finds.
Olshey will never get Cousins because every other GM is after him. Whitsitt found players without big names who were undervalued by other GMs. A GM's success depends on his ability as a talent evaluator of unknowns, not in his chasing obvious big names like Cousins. (Except for glittery Los Angeles, New York, and maybe Boston.)
(Looking at what I just wrote, I see that the 2nd paragraph is just a rewording of the 1st.)